Prior to the present invention, as shown by U.S. Pat. Nos. 4,055,705, Palmer et al and 4,095,003, Weatherly et al, superalloys used in the turbines of jet aircraft, were often coated with ceramic base materials, for example, blends of zirconium oxide and calcium oxide, which were plasma deposited onto the surface of the superalloy. In most instances, an intermediate metallic bonding layer, such as NiCrAlY, was applied onto the superalloy prior to spraying with a metal oxide. Although improved oxidation resistance was achieved, based on the use of such thermal barrier coating, spalling of the ceramic often resulted causing separation of the ceramic from the surface of the superalloy and rapid deterioration of the superalloy.
One of the principal causes of such spalling was the tendency of the zirconium oxide to transform its crystalline structure at elevated temperatures causing a breakdown of the adherent ceramic layer on the metal substrate. Efforts to reduce this tendency of the zirconium oxide to crystallize resulted in the use of yttrium oxide as a stabilizer in a zirconium oxide-yttria blend. Experience has shown, however, that under some conditions, vanadium impurities often present in turbine fuels may cause accelerated failure of these yttrium containing barrier coatings as a result of the reaction of the yttria component with vanadate salts present in salt deposits formed in the turbine to produce yttria reaction products and consequent destabilization of the zirconium oxide phase.
Zirconium oxide coatings containing calcium additives, such as calcium oxide or calcium silicate, are also subject to degradation in the presence of combustion gases containing sulfur dioxide because of the conversion of the calcium oxide stabilizer to gypsum calcium sulfate.
The present invention is based on the discovery that cerium oxide, or zirconium oxide stabilized with cerium oxide, resists the formation of such vanadate corrosion products thus rendering the ceramic coating more resistant to vanadium impurities over an extended period of time. In addition, such cerium oxide containing ceramic coatings are not affected by gases containing sulfur dioxide.
As a result, the application of cerium oxide or cerium oxide-zirconium oxide blends onto gas turbine hot section components offer a means for increasing parts lives and turbine operating efficiencies by providing increased corrosion resistance and/or reduced metal temperatures by the thermal barrier concept. The resulting ceramic coating is typically formed with an arc plasma spray generator, in which a continuous high power DC arc is used to ionize an inert gas such as argon or argon-hydrogen to produce a high temperature/high velocity plasma. Cerium oxide powder or blend thereof is injected into the plasma stream, heated to a molten or semi-plastic state and accelerated to a high velocity prior to impact on the alloy substrate being coated. In order to produce adherent coatings, a bond layer of MCrAlY is generally applied to the alloy surface prior to the formation of the coating, where M is a metal selected from iron, nickel or cobalt, or mixtures thereof.